1) Field of the Invention
The present invention relates to a signal receiving apparatus and a signal receiving system each of which is suitable for use in television receive-only (TVRO) stations each of which receives high-frequency signals from, for example, a communications satellite.
2) Description of the Related Art
In the satellite broadcasting communications, high-frequency signals transmitted from a signal transmitting station are received by a receive-only station via a communications satellite.
Such a satellite broadcasting communications service includes a BS (Broadcasting Satellite) broadcast, being broadcast communications for public, in which broadcast programs can be always received regardless of slots after a prescribed receiver's contract and CS (Communications Satellite) broadcast, being broadcast communications for corporations, in which limited broadcast programs can be received only for the slot after a receiver's contract effective for a fixed period of time. Different frequency bands are used for the BS broadcast and CS broadcast.
As to the CS communications satellites, four kinds of satellites including JCSAT-1, JCSAT-2, Superbird A, and Superbird B are well known. Each of the communications satellites transmits two kinds of 12-GHz-band high-frequency signals including a V-polarized wave high-frequency signal and an H-polarized wave high-frequency signal. This means that eight kinds of high-frequency signals are transmitted as broadcast high-frequency signals from the four CS-communications satellites.
The CS receive-only station receives only the high-frequency signals from the CS communications satellites. The CS receive-only station has, for example, the configuration shown in FIG. 13.
Referring to FIG. 13, the CS receive-only station 100 includes a dual antenna 101 and low-noise frequency converters (LNB: Low Noise Block-converter) 102A to 102D installed outdoors, together with amplifiers 103A to 103D, variable attenuators 106A to 106D, a variable switching box 107, and CS tuners 114a to 114f installed indoors.
The dual antenna 101, which receives four kinds of high-frequency signals among eight kinds of 12-GHz-band high-frequency signals transmitted from four satellites, has, for example, a caliber of 2.4 m.
Explanation will be made below on the case where the dual antenna 101 receives V-polarized and H-polarized wave high-frequency signals transmitted from each of JCSAT-1 and JCSAT-2 as four kinds of high-frequency signals.
Four LNBs 102A to 102D are arranged corresponding to respective four 12-GHz-band high-frequency signals received by the dual antenna 101. Each of the LNBs 102A to 102D down-converts the frequency of a different high-frequency signal respectively input thereto by a fixed frequency (e.g. 11.3 GHz) to obtain a 1-GHz-band signal.
For example, LNB 102A receives a V-polarized wave high-frequency signal from JCSAT-1. LNB 102B receives a H-polarized wave high-frequency signal from JCSAT-1. LNB 102C receives a V-polarized wave high-frequency signal from the JCSAT2. LNB 102D receives a H-polarized wave high-frequency signal from the JCSAT2.
Thus, the high-frequency signals input to the LNBs 102A to 102D are down-converted by a fixed frequency, respectively. Then the converted signals are respectively output to the amplifiers 103A to 103D while the noise component included in each converted signal is reduced.
Further, the amplifier 103A amplifies the signal down-converted by the LNB 102A. The amplifier 103B amplifies the signal down-converted by the LNB 102B. The amplifier 103C amplifies the signal down-converted by the LNB 102C. The amplifier 103D amplifies the signal down-converted by the LNB 102D. Each of the amplifiers 103A to 103D includes a booster amplifier 104 which amplifies the input signal and a booster amplifier power supply (booster amplifier PS) 105 which supplies electric power to the booster amplifier 104.
In such an arrangement, the signal transmission loss caused by the thickness and length of the transmission cable can be reduced by amplifying the signals from the LNBs 102A to 102D by means of the amplifiers 103A to 103D, respectively, while a signal degradation caused by the signal distribution in the variable switching box 107 at the rear stage can be reduced.
Further, the variable attenuator 106A attenuates the signal amplified by the amplifier 103A to a predetermined value to adjust finely the level of the signal input to the switching box 107. The variable attenuator 106B attenuates the signal amplified by the amplifier 103B to a predetermined value to adjust finely the level of the signal input to the switching box 107. The variable attenuator 106C attenuates the signal amplified by the amplifier 103C to a predetermined value to adjust finely the level of the signal input to the switching box 107. The variable attenuator 106D attenuates the signal amplified by the amplifier 103D to a predetermined value to adjust finely the level of the signal input to the switching box 107.
The variable switching box 107 switches to input signals from the variable attenuators 106A to 106D to predetermined CS tuners 114a to 114f, according to a signal receiver's contract held in the receive-only station 100.
The variable switching box 107 consists of distributors 108A to 108D which distribute respectively the signals input from the variable attenuators 106A to 106D to the CS tuners 114a to 114f, switches 109a to 109f which switch to turn on or off the signals distributed by each of the distributors 108A to 108D to the CS tuners 114a to 114f, and amplifiers 113a to 113f which respectively amplify the signals switched through the on/off operation of the switches 109a to 109f.
Further, the CS tuners 114a to 114f are receive tuners each of which receives a signal from each of the variable attenuators 106A to 106D switched by means of the variable switching box 107. In each of the CS tuners 114a to 114f, any one of the frequencies of four kinds of signals from the LNBs 102A to 102D is arbitrarily set as a receive frequency to receive the signal corresponding to the set receive frequency.
In the receive-only station 100 shown in FIG. 13, four CS tuners 114a to 114d are arranged as a working tuner. The CS tuners 114e and 114f are arranged as backup (spare) systems for the CS tuners 114a to 114d.
Here, the signal switching control of the variable switching box 107 to the CS tuners 114a to 114d will be described below.
For example, it is assumed that a receiver's contract is made on conditions that V-polarized wave signals transmitted from JCSAT-1 can be received for a predetermined period of time and that the signals are received with the CS tuner 114a among plural CS tuners 114a to 114d installed inside a building. In this case, a switch 109a is turned on to connect the variable attenuator 106A with the CS tuner 114a. The receive frequency of the CS tuner 114a is set to the frequency of the signal down-converted by means of the LNB 102A (to the frequency of the signal corresponding to the V-polarized wave signal of the JCSAT-1).
When it is desired that the CS tuner 114b installed at another place receives the V-polarized wave signal from JCSAT-1 during the period for which the receiver's contract is valid to receive it, a switch 109b is turned on to connect the variable attenuator 106B with the CS tuner 114b while the receive frequency of the CS tuner 114b is set to the frequency of the signal down-converted by the LNB 102A (to the frequency of the signal corresponding to the V-polarized wave signal of JCSAT-1).
When the period defined by the receiver's contract ends, the CS tuners 114a to 114d cannot receive the V-polarized wave signals from JCSAT-1.
In such a state, according to the communications satellite signal receiver's contract, the CS tuners 114a to 114d can receive four kinds of signals from the communications satellites by setting the receive frequency thereof under the signal switching control of the variable switching box 107.
In the same manner as that described above, when two communications satellites use high-frequency signals of the same frequencies (in other words, the same transponders are used), the CS tuners 114a to 114d can receive any one of four kinds of signals from the communications satellites under signal switching control of the variable switching boxes 107.
In the receive-only station 100 shown in FIG. 13, the CS tuners 114a to 114d are in service, whereas the CS tuners 114e and 114f are spare. Hence, the corresponding switches 109a to 109d and the amplifiers 113a to 113d are in service, whereas the switches 109e and 109f and the amplifiers 113e and 113f are spare.
In such an configuration, when each of the communications satellites JCSAT-1 and JCSAT-2 transmits V-polarized wave high-frequency signals and H-polarized wave high-frequency signals, the dual antenna 101 in the receive-only station 100 shown in FIG. 13 receives the four kinds of high-frequency signals transmitted. After the high-frequency signals are subjected to a predetermined process using the LNBs 102A to 102D, the amplifiers 103A to 103D, and the attenuators 106A to 106D, the processed signals are switched by means of the variable switching box 107. Then a predetermined one of the CS tuners 114a to 114d acting as a working tuner receives the switched one.
However, the problem is that since the variable switching box 107 in the receive-only station 100 shown in FIG. 13 includes four distributors 108A to 108D, the circuit configuration becomes large and complicated, thus it requires a considerable large installation space.
Moreover, many switches for signal switching operation arranged in the variable switching box 107 tends to cause erroneous switch-setting, slow switch setting, and occurrence of troubles such as failure. It cannot be ignored that maintenance costs of the variable switching box 107 is increased.
When the signal to be received by the CS tuners 114a to 114d is switched, the variable switching box 107 switches the signal while it must vary the set value of the receive frequency of each of the CS tuners 114a to 114d.
In this case, where the variable switching box 107 and the CS tuners 114a to 114d, for example, are installed at a different place in a building or where signals to be received by means of CS tuners 114a to 114d are necessary to be switched many times a day, both the switching operation of the variable switching box 107 and the setting of the receive frequency of each of the CS tuners 114a to 114d must be changed. Hence, the problem is that it is very troublesome to execute the receive signal switching operation.
In order to reduce such a trouble, it is considered that personal computers, for example, are respectively connected as monitoring control devices to the CS tuners 114a to 114d to monitor controllably them by remote control. However, connecting personal computers to the CS tuners 114a to 114d respectively results in an increased cost of the system construction.
In contrast, it may be considered that a sole personal computer, for example, is connected as the monitoring control device to the CS tuners 114a to 114d via a bus to monitor and control them by one operation.
However, in constructing such a system, an interface circuit needs to be arranged which connects the personal computer with the CS tuners 114a to 114d via a bus, in each of the CS tuners 114a to 114d. Hence, it is difficult to construct the system using existing CS tuners 114a to 114d. Like the above-described case, the problem is that the cost of the system construction is increased.
As described above, since the distributors 108A to 108D degrade the signals in the variable switching box 107, the amplifiers 103A to 103D arranged at the front stage, as shown in FIG. 13, amplify the signals, respectively. However, the problem is that arranging the amplifiers 103A to 103D leads to an increased cost.